Plugging the rear hole is not the solution. If you do that you have to cobble up or change your valve covers.

Chevy made a metal cap that fit into that hole for an early style pcv system. You can find the caps occasionally on Ebay or I suspect someone like Year One will have them. The cap had a provision for a vacuum hose. You run the hose to an in-line pcv valve and then onto the carb. You then should have a breather cap on your oil filler tube.

Another solution is to run the hose from the metal cap to the air cleaner to draw in air. Then, you have to change your oil filler tube to one that has a provision for a screw-in pcv. These are available through most Corvette parts vendors. A hose is then run from the pcv to the carb.

If you'd like further info, PM me and I'll try to send some pics.